There are numerous prior art signal limiting arrangements and these can be classified into a number of broad categories. One such category is that of breakdown devices, which are designed to protect the communication path from overvoltage conditions. These breakdown devices must pass voltage signals of one magnitude while blocking voltage signals of another magnitude. One such breakdown device is disclosed in U.S. Pat. application of Gordon, Mazurek and Wright, Ser. No. 764,594, filed Feb. 1, 1977, now U.S. Pat. No. 4,106,070, and is used to monitor the communication path for an overvoltage condition. Such a device is passive and only operates when such an overvoltage condition is detected. In that event, the device operates to open the communication path for the duration of the overvoltage condition. The difficulties encountered in utilizing such breakdown devices are: all signals on the communication path are blocked during the occurrence of the overvoltage condition; it is difficult to establish an accurate threshold level for triggering the breakdown device; and this form of signal limiting does not lend itself to limiting voice signals whose magnitude is small in relation to the ambient dc bias level appearing on the communication path.
A second category of signal limiting arrangements is that of signal attenuation circuits, which attenuate the signals appearing on the communication path when an overvoltage condition is detected. Essentially, this is a variation of the breakdown device category since, instead of opening the communication path when an excessive signal is detected, a variable impedance is employed to simply shunt the communication path. The difficulty with this arrangement is that while the excessive signal is attenuated, so are the weak signals also appearing on the communication path.
An alternative to the above-described passive signal limiting circuits is the active signal limiting arrangement disclosed in U.S. Pat. application of Angner, Egan, Gordon, and Huryn, Ser. No. 840,593, filed Oct. 11, 1977, now U.S. Pat. No. 4,145,579. The Angner et al signal limiting circuit employs a signal transformer which is physically constructed in a manner which allows the passage of voice frequency signals while preventing the passage of ringing and power frequency signals. The circuit is arranged to pass dial pulses and to shunt down the central office battery current in response to a detected high central office loop current condition which is indicative of a short loop. The difficulty with this arrangement is that all Touch-Tone network control signals are also attenuated.
Thus, prior art signal limiting arrangements operate somewhat indiscriminately, blocking or attenuating all signals, rather than exclusively controlling a particular excessive signal and passing all other signals unattenuated.